Complementary dual-modular redundancy memory cell

Abstract

A CDMR memory cell, includes a first bitcell which is used to store a current data level and a second bitcell which is used to store the complementary data level. When a read operation is performed, a comparator compares the data levels read from the two bitcells. If these two levels are not complementary, the comparator outputs an indicator. This indicator serves as an alert that a storage error has, or may have, occurred.

Claims

1. A memory cell, comprising: a data write input, configured to input data levels for storing in said memory cell; a data read output; an indicator output; a first bitcell connected to said data write input and said data read output, configured to store a first data level input from said data write input and to output a first read data level to said data read output; and a second bitcell connected to said data write input, configured to store a second data level input from said data write input to output a second read data level; and a comparator associated with said first bitcell, said second bitcell and said indicator output, configured to compare a data level read from said first bitcell and a data level read from said second bitcell and to provide, at said indicator output, an indicator when said first read data level and said second read data level are non-complementary, wherein said first bitcell comprises a write transistor and a read transistor, and wherein: a first diffusion connection of said write transistor is connected to said data write input, a gate connection of said write transistor is connected to a write trigger input, a first diffusion connection of said write transistor is connected to a first comparator input, a second diffusion connection of said write transistor is connected to a write trigger input, and a second diffusion connection of said write transistor is connected to a gate connection of said read transistor to form a first storage node; and said second bitcell comprises a write transistor and a read transistor, and wherein: a first diffusion connection of said write transistor is connected to said data write input, a gate connection of said write transistor is connected to said write trigger input, a first diffusion connection of said write transistor is connected to a second comparator input, a second diffusion connection of said write transistor connected to said write trigger input, and a second diffusion connection of said write transistor is connected to a gate connection of said read transistor to form a second storage node.

2. A memory cell according to claim 1, wherein said first and second data levels are input at said data write input in parallel.

3. A memory cell according to claim 1, further comprising at least one processor adapted to execute code instructions to provide said first data level to said first bitcell and a complementary data level to said second bitcell.

4. A memory cell according to claim 1, further comprising an inverter connected to said data write input, configured to invert said first data level and to provide said inverted data level as said second data level to said second bitcell.

5. A memory cell according to claim 1, further comprising at least one processor adapted to execute code instructions to determine, in accordance with said indicator, a correct data level from at least one of said read data levels.

6. A memory cell according to claim 5, further comprising a parity bit, and said code instructions for determining a correct data level use said parity bit for said correcting.

7. A memory cell according to claim 1, wherein said memory cell is a static memory.

8. A memory cell according to claim 1, wherein said memory cell is a dynamic memory.

9. A memory cell according to claim 1, wherein write operations for said first and second bitcells are triggered from a write trigger input and read operations for said first and second bitcells are triggered from a read trigger input.

10. A memory cell according to claim 1, wherein said first bitcell and said second bitcell comprise respective write transistors triggered from a write trigger input and respective read transistors triggered from a read trigger input.

11. A memory cell according to claim 10, wherein write transistors of said first and second bitcells are a same transistor type.

12. A memory cell according to claim 10, wherein read transistors of said first and second bitcells are a same transistor type.

13. A memory cell according to claim 10, wherein all transistors of said first and second bitcells are a same transistor type.

14. A memory cell according to claim 10, wherein all transistors of said first and second bitcells are gain cell embedded DRAM (GC-eDRAM) transistors.

15. A memory cell according to claim 1, wherein said first bitcell comprises a read bit line output configured to output said first read data level read from said first bitcell.

16. A memory cell according to claim 1, wherein all of said first bitcell transistors and said second bitcell transistors are gain cell embedded GC-eDRAM transistors.

17. A memory array comprising: a data write input port, configured to input data levels for storing in said array; a data read output port, configured to output data levels from said memory array; an indicator output port; a plurality of memory cells, each of said memory cells comprising: a first bitcell connected to said data write input port and said data read output port, configured to store first data levels and to output first read data levels; and a second bitcell connected to said data write input port, configured to store second data levels and to output a second read data levels; at least one comparator, configured to compare respective first and second read data levels a selected memory cell and to provide, at said indicator output port, a respective indicator when said first read data level and said second read data level are non-complementary; and at least one non-transitory computer readable storage medium storing instructions and at least one processor configured to execute said instructions to correct memory cell errors by applying error correction logic to at least one data level read from a respective one of said memory cells and a respective indicator.

18. A memory array according to claim 17, wherein said processor is further configured to execute said instructions to provide, at said write input port, first data levels for storing in said first bitcells and complementary data levels for storing in respective second bitcells.

19. A memory array according to claim 18, further comprising a parity array comprising at least one parity bit per column of said memory cells, wherein said error correction logic uses said parity array for said correcting said memory cell read errors.

20. A memory array according to claim 18, wherein said memory array is a static memory array.

21. A memory array according to claim 18, wherein said memory array is a dynamic memory array.

22. A memory array according to claim 18, wherein said first bitcells and said second bitcells are stored in different rows of said memory array.

23. A memory array according to claim 18, wherein said first bitcells and said second bitcells are stored in different portions of said memory array.

24. A method comprising: providing a memory cell comprising: a first bitcell configured to store a first input data level and to output said stored first data level to a data read output; and a second bitcell configured to store a second input data level; storing a first data level in said first bitcell and a complementary data level in said second bitcell; reading first and second data levels from said first and second bitcells respectively; providing an indicator when first and second read data levels are non-complementary; performing error correction on said first and second read data levels to correct said first data level when said indicator is provided; and outputting said first data level.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced.

(2) In the drawings:

(3) FIG. 1 illustrates a particle strike hitting a silicon substrate junction;

(4) FIG. 2A is a simplified layout of a DICE bitcell;

(5) FIG. 2B is a simplified layout of a 13T radiation hardened SRAM;

(6) FIG. 2C is a simplified layout of a SHIELD SRAM;

(7) FIG. 3A is a simplified block diagram of a CDMR memory cell, according to embodiments of the invention;

(8) FIG. 3B is a simplified block diagram of a CDMR memory cell with read/write triggering, according to embodiments of the invention;

(9) FIG. 3C is a simplified layout of a four transistor PMOS CDMR memory cell, according to embodiments of the invention;

(10) FIG. 3D is a simplified layout of a four transistor NMOS CDMR memory cell, according to embodiments of the invention;

(11) FIG. 4 is a waveform demonstration of write-upset-read events;

(12) FIG. 5 is a simplified block diagram of a memory array of 4T CDMR memory cells, according to embodiments of the invention;

(13) FIG. 6A is a simplified circuit diagram of a CDMR memory cell with differential sense amplifier, according to exemplary embodiments of the invention;

(14) FIG. 6B is a simplified circuit diagram of a CDMR memory cell with single-ended readout with error detection, according to exemplary embodiments of the invention;

(15) FIG. 6C is a histogram showing read access time distribution for single-ended readout;

(16) FIG. 6D is a histogram showing read access time distribution for differential readout;

(17) FIG. 7 is a graph showing storage node degradation of a simulated 4T CDMR memory cell following a write operation under worst case WBL bias conditions;

(18) FIG. 8 is a histogram showing DRT distribution of a simulated 4T CDMR memory cell; and

(19) FIG. 9 is a graph comparing the retention power of a simulated 4T CDMR memory cell and prior art solutions;

(20) FIG. 10A is a graph of the refresh rate of a simulated 4T CDMR memory cell;

(21) FIG. 10B is a graph of the refresh power consumption of a simulated 4T CDMR memory cell; and

(22) FIG. 11 is a graph of critical charge as a function of the time after write.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

(23) The present invention, in some embodiments thereof, relates to a CDMR memory cell and, more particularly, but not exclusively, to a 4-transistor CDMR memory cell.

(24) Embodiments of the CDMR memory cell presented herein include two bitcells which store complementary data levels. One of the bitcells holds the actual data level being stored and the other bitcell holds the complementary data level. When a read operation is performed, a comparator compares the data levels read from the two bitcells contained in the CDMR memory cell. If these two levels are not complementary, the comparator outputs an indicator which serves as an alert that a storage error has (or may have) occurred. Further error detection and/or error correction techniques may then be employed on the data read from the CDMR memory cell.

(25) In the most general case, the indicator does not detect storage errors with 100% probability. If an error occurs in both bitcells, the output levels will still be complementary and no error will be indicated. However, for some transistor types an error in both bitcells is unlikely, and relatively simple error detection and correction logic may be used to determine the correct data level.

(26) As used herein the terms “complementary dual-modular redundancy memory cell”, “CDMR memory cell” and “memory cell” mean a one-bit memory which stores two data level values, where each data level may be read independently without changing the other data level. Optionally the two data levels are stored in respective bitcells.

(27) As used herein the term “bitcell” means a one-bit memory.

(28) As used herein the terms “complementary data level” means the data level of the opposite logic value. For example, if the data level stored in the first bitcell corresponds to a logic “0” the complementary data level corresponds to a logic “1”.

(29) Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and/or methods set forth in the following description and/or illustrated in the drawings and/or the Examples. The invention is capable of other embodiments or of being practiced or carried out in various ways.

(30) I. CDMR Memory Cell

(31) Referring now to the drawings, FIG. 3A is a simplified block diagram of a CDMR memory cell, according to embodiments of the invention. CDMR memory cell 300 includes two bitcells, 310 and 320, and comparator 330. For correct operation of the CDMR memory cell, the data level of the data actually being stored is written to bitcell 310 and the complementary data level is written to bitcell 320.

(32) When a read operation is performed, comparator 330 compares the data level read from said first bitcell to the data level read from the second bitcell. Comparator 330 outputs an indicator which shows if the two read data levels are or are not complementary. If the data level in a single bitcell has flipped, the indicator will show that the data levels are not complementary. Optionally, an indicator showing non-complementary levels serves as an alert that error detection and/or error correction and/or other operations should be performed.

(33) Optionally, the comparator is an XOR gate which compares logic levels read from memory cell. Alternately, the comparator is an analog comparator which compares analog levels output from the memory cell.

(34) Comparator 330 may be located adjacent to the bitcells, or may be located in a different location on the circuit (e.g. ASIC).

(35) Optionally, the timing of the read and/or write operations are controlled to ensure that in each read cycle corresponding data levels are provided to comparator 330 for comparison (so that the data level written to bitcell 320 was complementary to the data written to bitcell 310 and therefore should be complementary at read time).

(36) Optionally, the data write input includes separate input lines for bitcells 310 and 320, and the complementary data levels are provided to the memory cell in parallel to be stored in the memory cell in a single cycle. Alternately, the complementary data levels are provided to memory cell 300 in series via a single input line and stored in the respective bitcell in separate cycles. Optionally, CDMR memory cell 300 includes an inverter. In the exemplary embodiment shown in FIG. 3B, inverter 340 inverts the logic level at WBL, thereby obtaining the complementary data level, and provides the complementary data level to bitcell 320. There is then no need to provide the complementary data level separately to WBLB.

(37) Optionally, based on the indicator, error detection and/or correction operations are performed on the data levels read from one or both of bitcells 310 and 320. The memory detection and/or correction operations may be performed by any suitable techniques known in the art.

(38) Optionally, CDMR memory cell 300 includes, or is associated with, a parity bit and the error detection and/or correction operations use the parity bit. Optionally, the parity bit is stored in a parity bitcell which is integrated in CDMR memory cell 300. Alternately or additionally, the parity bit is stored externally to CDMR memory cell 300.

(39) Optionally, the parity bit is stored in a separate CDMR memory cell. Alternately or additionally, the parity bit is stored in a different type of bitcell.

(40) Optionally, other error detection and/or correction techniques known in the art are used for a single CDMR memory cell or for a memory array formed from multiple CDMR memory cells. These techniques may include using parity bit(s), data replication, error correction coding and so forth.

(41) Optionally, CDMR memory cell 300 is a dynamic memory cell. Alternately, CDMR memory cell 300 is a static memory cell.

(42) Reference is now made to FIG. 3B, which is a simplified block diagram of a CDMR memory cell with read write triggering, according to an exemplary embodiment of the invention. CDMR memory cell 305 includes two bitcells and a comparator configured as shown above, and additionally includes write and read trigger inputs (WWL and RWL respectively).

(43) Bitcell 310 stores the data level which is at the data write input (WBL) during a write operation. Bitcell 320 stores the data level which is at the complementary data write input (WBLB) for the same write operation. Write operations to bitcells 310 and 320 are triggered by a common write trigger input (WWL). Read operations from bitcells 310 and 320 are triggered by a common write trigger input (RWL). The data read from bitcell 310 is output at data read output RBL. Optionally, CDMR memory cell 305 also includes a second data read output (RBLB) for bitcell 320.

(44) When the CDMR memory cell does not include inverter 340 and the write operation is triggered by a common trigger (WWL), the data levels at WBL and WBLB should be complementary at the time of the write operation.

(45) Optionally, write and read operations to CDMR memory cell 305 are performed in parallel.

(46) Alternately, write and/or read operations to CDMR memory cell 305 are performed in series. Optionally, for parallel write operations, bitcells 310 and 320 are triggered independently by respective write triggers (i.e. not by common write trigger WWL as shown in FIG. 3B). Optionally, for parallel read operations, bitcells 310 and 320 are triggered independently by respective read triggers (i.e. not by common read trigger RWL as shown in FIG. 3B).

(47) Optionally, bitcells 310 and 320 are two-transistor (2T) bitcells which include respective write and read transistors. Further optionally, write operations to the two write transistors are triggered together by a write trigger (e.g. WWL in FIG. 3B) and read operations from the two read transistors are triggered together by a read trigger (e.g. RWL in FIG. 3B).

(48) Optionally the two write transistors are the same transistor type. Alternately or additionally, the two read transistors are the same transistor type.

(49) Optionally, all four transistors are the same transistor type. Further optionally, all four transistors are gain cell embedded DRAM (GC-eDRAM) transistors.

(50) Different embodiments of the CDMR memory cells may use different types of transistors. Optional types of transistors which may be used to construct the CDMR memory cell include, but are not limited to:

(51) i) Ultra-high Vt;

(52) ii) High Vt;

(53) iii) Regular Vt;

(54) iv) Low Vt;

(55) v) Ultra-low Vt;

(56) vi) Native; and

(57) vii) I/O NMOS/PMOS.

(58) Embodiments of CDMR memory cells may be implemented in circuits, including, but not limited to:

(59) a) An integrated circuit (IC) customized for a particular use, such as an Application-Specific Integrated Circuit (ASIC);

(60) b) A programmable logic device intended for general-purpose use. Examples of such programmable logic devices include, but are not limited to: Field-Programmable Gate Array (FPGA), Gate Array, Uncommitted Logic Array (ULA), Programmable Logic Array (PLA), Programmable Array Logic (PAL), Complex Programmable Logic Device (CPLD), Erasable Programmable Logic Device (EPLD) and Structured ASIC.

(61) II. Four Transistor (4T) CDMR Memory Cell

(62) Optionally, the CDMR memory cell is a four-transistor (4T) memory cell which includes two 2T bitcells and a comparator. In the 4T embodiment, each of the bitcells includes a respective write transistor and a respective read transistor.

(63) Reference is now made to FIG. 3C, which is a simplified layout of a four transistor (4T) PMOS CDMR memory cell, according to exemplary embodiments of the invention. The 4T CDMR memory cell includes two write transistors (MW1, MW2) and two read transistors (MR1, MR2). The two read data levels, at RBL and RBLB, are provided to the comparator (not shown).

(64) FIG. 3D is a simplified layout of a 4T NMOS CDMR memory cell, according to exemplary embodiments of the invention. The layout of the NMOS CDMR memory cell is very similar to the NMOS layout, and includes two write transistors (MW1, MW2) and two read transistors (MR1, MR2) interconnected as shown in FIG. 3D.

(65) During normal operation, the output of a read operation from the MR1 and MR2 would provide two opposite levels. Therefore, if both storage nodes provide the same read out level, it may be concluded (or suspected) that a read error has occurred.

(66) The data and its complementary values are stored on the parasitic capacitances at the storage nodes (SN and SNB), comprising the gate capacitance of MR1/MR2 and the diffusion capacitance of MW1/MW2, respectively. Writing to the cell is performed by driving the write word line (WWL) to a negative voltage and passing the data and its complementary level from the write bit lines (WBL and WBLB) to SN and SNB, respectively. Readout is performed by pre-discharging the read bit lines (RBL and RBLB) to GND and driving the read word lines (RWL and RWLB) to VDD, thereby charging RBL/RBLB only if SN/SNB holds a data ‘0’. The RBL/RBLB of each column is connected to a sensing circuit, which may be implemented with a simple inverter, in order to output the digital levels of the data and its complementary level stored in the selected cell.

(67) III. CDMR Memory Array

(68) Optionally, CDMR memory cells are combined to form a CDMR memory array. The memory array architecture may be any architecture known in the art that is suitable for the type of transistors forming the CDMR memory cells. An exemplary embodiment of a memory array architecture is illustrated in FIG. 5 below.

(69) The CDMR memory array includes multiple CDMR memory cells, each of which includes respective first and second bitcells for storing complementary logic levels. The CDMR memory array further includes one or more comparators which compare the data levels read from the first and second bitcells of a respective CDMR memory cell, and provides respective indicators showing when the read data levels are non-complementary.

(70) There are multiple options for positioning the respective bitcell pairs within the memory array. Optional arrangements include:

(71) i) The memory array is laid out as an array of CDMR memory cells, with the first and second bitcells interconnected within the respective memory cell.

(72) ii) The first bitcells and the second bitcells are located on different rows of the memory array.

(73) iii) The first bitcells and the second bitcells are located in different portions of the memory array.

(74) iv) The first bitcells and the second bitcells are located on separate sub-arrays.

(75) Optionally, the memory array is read in multi-bit data words along with a corresponding indicator word having a respective indicator for each bit in the data word.

(76) Optionally, write bit line inputs of the memory cells are connected to form a common write bit line and respective complementary write bit line inputs of the memory cells are connected to form a common complementary write bit line.

(77) In itself, the CDMR characteristic of the memory cells in the memory array provides a reduced area implementation with inherent error detection capabilities. However, while the location of the error is known, it is not inherently clear if the data or its complementary value was corrupted, which may prevent correction of the error. This may be addressed by adding a parity bit to each set of bits. If the parity is maintained despite the error, the complementary value of the erroneous bit (SNB) has been corrupted and should be corrected, while if the parity is incorrect, the SN value should be corrected.

(78) Optionally, the CDMR memory array includes a parity array which stores at least one parity bit per column of CDMR memory cells. Error correction logic may use the parity array for the correcting the memory cell read errors.

(79) Optionally, the CDMR memory array according to claim includes at least one processor which applies error correction logic to the data levels read from the CDMR memory cell in the array along with the respective indicators obtained from the comparator(s).

(80) Optionally the CDMR memory array is a static memory. Alternately, the CDMR memory array is a dynamic memory.

(81) IV. Exemplary 4T GC-eDRAM CDMR Memory Cell

(82) Optionally, the 4T CDMR memory cell uses GC-eDRAM transistors. A 4T GC-eDRAM CDMR memory cell is robust against SEUs and data deterioration errors.

(83) In general, the primary risk of an SEU triggered data flip in a conventional SRAM cell is due to the positive feedback between the two internal storage nodes. The SRAM structure is highly susceptible to a particle strike, as any voltage shift which causes one of the storage nodes to cross the switching threshold of the adjacent inverter will result in a bit flip.

(84) Gain-cell eDRAM (GC-eDRAM) is a fully logic-compatible implementation of eDRAM, which provides a reduced silicon footprint as compared to SRAM, but lacks the internal feedback that ensures strong storage levels, in spite of deteriorating leakage currents. Intuitively, such a topology is much more susceptible to SERs, as the circuit lacks any mechanism to mitigate a level change induced by a particle strike. However, with complementary dual-modular redundancy both the data value and its inverse (i.e. complementary) value are stored for each bit. Based on this concept, SEUs in the CDMR memory cell or a CDMR memory array may be both detected and corrected, while still achieving the low area and power aspirations of the target applications.

(85) The 4T GC-eDRAM CDMR memory cell lacks the internal feedback mechanism of the SRAM cell. Therefore, the complementary storage nodes (SN and SNB) are affected separately by a particle strike. In the case of a particle changing the data in one of the storage nodes, the complementary node will remain unaffected and both nodes will store the same data level. Therefore, by simply comparing the outputs (e.g. with an XOR gate) an error may be detected in any given bit.

(86) For example, in the all-PMOS cell of FIG. 3C the only nodes susceptible to a strike are the reverse-biased p+n junctions at SN and SNB. For such a junction, only a positive voltage shift can occur due to a particle strike, and therefore only a stored data ‘0’ may be flipped. If the particle strike occurs just after a write ‘0’ operation to the cell, the level might get deteriorated and its retention time would get decreased. On the other hand, a particle strike close to the retention time of the cell will quite surely flip its state and cause an error during readout. On the other hand, a particle strike on a node storing a data ‘1’ would only strengthen the stored level, since MR1 would be more cut off during a readout. Furthermore, its retention time would be increased as a result of the positive voltage shift.

(87) The operation of the 4T GC-eDRAM CDMR memory cell under SEUs is demonstrated in FIG. 4. FIG. 4 shows consecutive write, upset, and read events for data ‘1’ and ‘0’ with a 400 mV supply voltage. The SEU was modeled by connecting a current source to the SN node of the bitcell and applying a double-exponential current pulse during the standby states of the cell, according to the model presented in [18]. The physical composition of the cell only allows a positively charged upset [15], and therefore, in the example, only this type of particle strike is shown. In the first demonstrated strike (particle strike 1), the cell is storing a ‘1’, and the positive charge only strengthens the stored level, leaving RBL discharged during readout and not leading to an error. On the hand, when data ‘0’ is stored, the applied pulse (particle strike 2) causes an increase in the voltage level stored at SN, causing the RBL to erroneously charge during readout. Since the complementary value, stored in SNB, is also a data ‘1’, both RBL and RBLB provide a logic ‘1’ at the output, indicating that an error has occurred. In addition, the parity will no longer be correct, which implies that the error occurred in the SN of the erroneous bit, and therefore, this node should be pulled back down to ‘0’ to correct the error.

(88) The sensitivity of a memory bitcell to particle strikes is commonly quantified using the C.sub.crit metric, which is determined by integrating the applied current source at the point where it causes a read failure following an SEU. However, the inherent error detection and correction capabilities of the proposed topology and architecture significantly reduce the relevance of the C.sub.crit metric and make it inappropriate for comparison with other solutions. Nonetheless, the C.sub.crit of the 4T bitcell was extracted through simulation under particle strikes, as described in Sec. V-D below.

(89) V. Exemplary CDMR Memory Array

(90) Reference is now made to FIG. 5, which is a simplified block diagram of a CDMR Memory Array, according to embodiments of the invention. It is noted that the invention is not limited to a particular type of memory cell, or memory array size, architecture and/or structure. The CDMR memory array may be implemented in any size or architecture which incorporates CDMR memory cells as described herein. The analysis and results presented below are for a CDMR Memory Array comprising a 64×32 array of 4T GC-eDRAM CDMR memory cells.

(91) Optionally, in order to enable recovery of the correct level from a flipped cell at least one parity column is integrated to the CDMR memory array. These parity cells hold the parity bit of the corresponding word. For the 4T GC-eDRAM CDMR memory cell, the correct data may be recovered from an error detected cell using simple comparison logic. In general, any number of errors may be recovered from the accessed row depending on the number of parity columns. Every parity bit enables the recovery of an additional error, compromising area overhead due to additional parity bitcells. These parity cells are optionally 4T CDMR memory cells.

(92) For proof of concept, the results presented below used a single parity bit in every row consisting of 32 bits, corresponding to a single error recovery per row, resulting in an area overhead of less than 3%.

(93) A. Single-Ended and Differential Readout Schemes

(94) One of the drawbacks of conventional GC-eDRAM arrays is their relatively slow read access time due to a single ended readout scheme.

(95) In contrast, embodiments of the 4T CDMR memory cell produce both BL and BLB signals, corresponding to the two opposite level stored in the cell. Therefore, a differential readout scheme may be employed, resulting in a faster access time and reliability. To illustrate this, the read access time of a single ended and differential readout schemes was post-layout simulated and compared.

(96) For the comparisons presented below, a conventional latching sense amplifier was used for the differential readout mode, while a simple inverter was used for the single-ended readout mode. Only minimal sized regular V.sub.T transistors were used. The differential sense amplifier circuit and single-ended readout with error detection circuits are shown in FIGS. 6A and 6B respectively.

(97) FIGS. 6C and 6D are histograms showing the read access times of single-ended readout mode and differential readout mode respectively. The read access times were extracted from 1K Monte-Carlo simulations modeling both global and local variations (mismatch) under a 700 mV supply voltage. As seen in FIGS. 6C-6D the differential sense amplifier has a faster access time. Both readout schemes may also be used independently without affecting the RBL level, due to the high input impedance of both sense circuits. While the differential readout provides better performance, the two single ended sense inverters may be used to produce delayed results, indicating whether an error has occurred.

(98) VI. Comparison Results

(99) In order to demonstrate the advantages of the 4T GC-eDRAM CDMR memory cell over conventional 6T SRAM and other error tolerant solutions, section VI presents CDMR memory cell layout and error tolerance comparisons.

(100) A. Layout Comparison

(101) One of the most important factors in memory design is a small bitcell area, in order to be able to integrate as much memory as possible with a small area cost. As a result, conventional SRAM memories are usually implemented in dense design rules, trading off stability and soft error tolerance. Therefore, one of the main goals in the design bitcells is to maintain a small area penalty, while still maintaining a soft error tolerance.

(102) The layout of a 65 nm 4T CDMR memory cell, implemented with standard-VT PMOS transistors, comprises an area of 1.01 μm.sup.2. The CDMR memory cell is 47% smaller than the standard non-SEU tolerant memory circuit, and between 2.5×-5× smaller than the other considered bitcells (see Table 1 below).

(103) In an exemplary embodiment of the 4T GC-eDRAM CDMR memory array, the parity bits are stored in 4T GC-eDRAM CDMR memory cells and therefore they consume less than 3% of the total array size. Use of a single parity cell enables the error correction of one bit per row. Additional parity cells may enhance the error correction capabilities, therefore presenting a trade off with the resulting array size as described below. The extra periphery needed for error correction is implemented using an additional AND gate per every two columns and standard error correction circuitry, used in conventional ECCs.

(104) B. Error Detection and Correction

(105) State-of-the-art circuit level solutions, such as DICE, Quatro 10T and SHIELD, are mainly targeted at high-radiation environments such as space, where memories are likely to encounter SEUs caused by alpha particle strikes. While these memories are more immune to SEUs, they do not detect any number of errors in case an SEU has occurred. Moreover, errors caused by process, voltage, or temperature (PVT) variations cannot be detected or corrected. In order to be able to detect errors, ECC must be integrated into the memory array as well, requiring even more area and complexity to the already modified memory. Nonetheless, ECCs can only detect a limited number of errors based on the used ECC scheme, as described in section II-B.

(106) In contrast, the CDMR memory array presented herein may detect all the errors of an accessed row, independent of the number of bits per row and without the need for extra hardware. This may be done using the dual single-ended readout structure, indicating an error has occurred if both outputs are ‘1’, as described in section III-D. The error correction capability depends on the amount of extra bitcells holding the parity bits of the row. For example, the addition of two parity cells provides a capability of correcting up to two errors per row, similar to single error correction-double error detection (SECDED) and at a much smaller area overhead.

(107) V. Simulation Results

(108) A. Data Retention Time Analysis

(109) The data retention time (DRT) of GC-eDRAMs is a dominant factor in the memory design, affecting both the power consumption and the availability of the memory array. It is primarily limited by the level set by the initial charge stored in the bitcell and the leakage currents that degrade this level. After write, the stored data starts deteriorating due to leakage currents from the storage node, and therefore it requires periodic refresh cycles. Depending on the type of write transistor (WT), one of the data levels has a much higher retention time than the other (‘1’ for a PMOS WT, ‘0’ for a NMOS WT). However, when determining the refresh frequency, one must consider the deterioration of the weaker data level under worst-case conditions, i.e. when the write bitline (WBL) is driven to the opposite level of the stored data during retention periods.

(110) The exemplary 4T all-PMOS CDMR memory cell presented herein, displays asymmetric retention characteristics with highly advantageous retention of data ‘1’ over data ‘0’. This is illustrated in FIG. 7, which shows the storage node degradation following a write operation under worst case WBL bias condition. The plot was extracted from 1K Monte-Carlo simulations modeling both process variations and mismatch. The result of the above phenomenon indicate that a read failure can only occur due to a deteriorated ‘0’ level in the cell. The DRT of the 4T cell was determined as the time after write when the voltage difference between the degraded ‘1’ and ‘0’ levels deteriorates to 200 mV. The resulting DRT distribution is given in FIG. 8, with a worst case DRT of 50 us.

(111) B. Static Power Consumption

(112) The static power of memory arrays is often considered the most important aspect of chip power consumption, due to the large number of bitcells in a standby state at any given time. Whereas leakage is the dominant static power component of a standard SRAM, when considering a dynamic memory such as the CDMR memory cell, the static power comprises both the leakage and the refresh power of the array, and in most cases, the refresh power is the dominant factor. Since the refresh power is a function of the data retention time (DRT), extraction of the DRT is essential for static power estimation.

(113) The standby power of a GC-eDRAM comprises the leakage power of the array and the dynamic refresh energy, consumed during every refresh period. This total standby power is often referred to as the retention power of the array. For correct power analysis, the refresh period for the simulation was chosen to be 50 μs, and the reciprocal of this value was used as the refresh rate in calculation of the refresh power.

(114) FIG. 9 compares the retention power of the exemplary 4T GC-eDRAM CDMR memory cell with the static power consumption of a conventional 6T SRAM cell and other circuit level SEU hardened bitcells. The refresh power constituted 40% to 60% of the total power consumption of the 4T GC-eDRAM CDMR memory cell, depending on the supply voltage. Simulations were made under worst-case biasing during retention with WBL and WBLB kept at the opposite voltage levels to those stored at SN and SNB, resulting in the highest leakage currents. Nevertheless, the 4T GC-eDRAM CDMR memory cell clearly has the lowest standby power consumption across the entire range of simulated supply voltages. This is a reduction of 59%-87% at nominal conditions (1.2 V) and 48% lower than the SHIELD solution at 0.4V, which is the only other functional solution at such a scaled operating voltage.

(115) C. Refresh Power Reduction Using Error Correcting Scheme

(116) The error correcting nature of embodiments of the GC-eDRAM CDMR memory cell array relies on the fact that only data ‘0’ is subject to errors. This is due to several factors. Since a particle strike may only cause a positive voltage shift on the storage nodes of the cell, SEU can only affect data ‘0’, causing a readout failure. Furthermore, as discussed in section VI-A, data ‘0’ is the weaker level in terms of retention time characteristics, making it susceptible to errors due to data deterioration resulting from leakage currents. As a result, the power consuming refresh cycles rate is set by the DRT of data ‘0’.

(117) This phenomenon indicates that using the error correcting scheme of the memory array and lowering the refresh rate, allowing a number of possible errors which may be corrected, may result in substantial power savings. As previously discussed, the number of allowable errors is set by the number of parity bits per row, corresponding to the error correcting capability of the memory array. FIG. 10A shows the refresh rate as a function of the parity bits per row, indicating that a single parity bit may reduce the refresh rate by almost two times, while extending the number of parity bits to 4 results in a 2.5 times reduction in the refresh rate. This results in substantial power reduction, illustrated in FIG. 10B, where the power consumption per cycle for a 2 Kb array is reduced by almost 3 times using 4 parity bits.

(118) D. Critical Charge

(119) The critical charge is a commonly used design metric in determining the tolerance level of a cell to SEU, occurring due to a particle strike at a cell's internal node. While the CDMR memory array error detecting and correcting schemes enable the recovery of an error due to a SEU, for fair comparison to other bitcell solutions, this section will analyze the 4T GC-eDRAM CDMR memory cell critical charge and compare it to other SEU hardened bitcells. In order to mimic the event of a particle strike, SEUs are modeled by injecting the current pulse at the sensitive nodes. The current pulse has fast rise time and gradual fall time. The shape of the current pulse is approximated by:

(120) $\begin{array}{cc}I\left(t\right)=\frac{Q_{\mathrm{coll}}}{t_f-t_r}\left(e^{-\frac{t}{t_f}}-e^{-\frac{t}{t_r}}\right),& \left(1\right)\end{array}$
where Q is the charge collected due to particle strike, t.sub.r is the rise time and t.sub.f is the fall time. The sensitivity of a device described by the minimum charge needed to upset the function. This minimum charge is called the critical charge (Q.sub.crit). The simplest approach is to consider the critical charge as the product of the total capacity C.sub.i at a given node by the power supply voltage V.sub.DD, therefore, Q.sub.crit=C.sub.i×V.sub.DD. This approach is very useful to obtain a rough estimation of the critical charge. A more precise approach to obtain the critical charge is to use a circuit simulator and to describe the collected charge by a double exponential current generator of short duration applied at the sensitive node. The total charge deposited by a particle strike may be calculated by numerically integrating the transient current pulse, and Q.sub.crit is defined as the minimum charge deposited in a sensitive node that results in a memory bit-flip.

(121) In the case of the 4T GC-eDRAM bitcell, a bit flip may only be determined at the point where a read operation fails, similarly to the calculation of the retention time of the cell. Moreover, the critical charge varies with time due to the deterioration of the stored voltage in the cell, specifically for data ‘0’, which is susceptible to SEU. Simulation of a particle strike with the SEU model of (1) is the most popular technique for evaluation of the rad-hardening ability of a memory bitcell. For the 4T GC-eDRAM CDMR memory cell, the critical charge was simulated in this method at different times after a write operation, and determined at the point where a readout failure was detected following the injected current pulse. The results are shown in FIG. 11 for a under GC-eDRAM CDMR memory cell worst-case retention time conditions. As expected, the critical charge has the highest value following a write ‘0’ operation and it degrades as time passes due to a deteriorated ‘0’ level as a result of leakage. After 50 us the critical charge is equal to 0 since the retention time has passed and any particle strike may cause a readout error. While these values are significantly smaller than those presented by other circuit level solutions, the error-correction and detection capabilities of the cell make them sufficient for high-radiation operation.

(122) Table 1 summarizes the comparison between the 4T CDMR memory and other considered solutions. The table clearly emphasizes the benefits of the proposed solution, achieving much lower static power and area consumption compared to other memory options and including inherent error detection capability and a low overhead error correction capability.

(123) TABLE-US-00001 TABLE 1 Comparison of Memory Solutions for Space Applications Memory type 6T SRAM 12T DICE Quatro 10T SHIELD 4T CDMR memory Cell size 451 f.sup.2 861 f.sup.2 1159 f.sup.2 947 f.sup.2 240 f.sup.2 Cell static power 54.9 pW 58.3 pW 110.6 pW 35.7 pW 14.7 pW @ 1.2 V Protection — Increased Increased Increased CDMR mechanism C.sub.crit C.sub.crit C.sub.crit Error detection requires parity or ECC for each detection inherent per-bit detection Error correction requires ECC (e.g., 7 bits for 1 error in 32-bit word) 1-bit parity per error

(124) In summary, some embodiments of the invention described herein provide error tolerant CDMR memory cells and CDMR memory arrays, optionally based on GC-eDRAM transistors. Both circuit and architectural techniques may be used to provide a soft error tolerant memory. Embodiments of a CDMR memory array formed from CDMR memory cells are fully compatible with low voltage operation, consume less area than any other soft-error tolerant solution and are capable of detecting any number of errors during a readout operation. Furthermore, multi-bit error correction may be done with only a modest increase in the area overhead. In addition, the CDMR memory array is compatible with differential readout schemes, resulting in a better performance and reliability than conventional GC eDRAM arrays.

(125) It is expected that during the life of a patent maturing from this application many relevant transistors, types of transistors, bitcells, memory arrays, memory array architectures, error detection techniques and error correction techniques will be developed and the scope of the terms transistor, transistor type, bitcell, memory array, memory array architecture, error detection and error correction are intended to include all such new technologies a priori.

(126) The terms “comprises”, “comprising”, “includes”, “including”, “having” and their conjugates mean “including but not limited to”.

(127) The term “consisting of” means “including and limited to”.

(128) The term “consisting essentially of” means that the composition, method or structure may include additional ingredients, steps and/or parts, but only if the additional ingredients, steps and/or parts do not materially alter the basic and novel characteristics of the claimed composition, method or structure.

(129) As used herein, the singular form “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “a compound” or “at least one compound” may include a plurality of compounds, including mixtures thereof.

(130) Throughout this application, various embodiments of this invention may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

(131) Whenever a numerical range is indicated herein, it is meant to include any cited numeral (fractional or integral) within the indicated range. The phrases “ranging/ranges between” a first indicate number and a second indicate number and “ranging/ranges from” a first indicate number “to” a second indicate number are used herein interchangeably and are meant to include the first and second indicated numbers and all the fractional and integral numerals therebetween.

(132) It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements.

(133) Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

(134) All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. To the extent that section headings are used, they should not be construed as necessarily limiting.